For flying, I wonder if a dismantle-able (DIY/MYOG) metal protective cage could be made from metal (aluminium?) angle and some M5 nuts and wingnuts; the rectangular-'cube' would have X-reinforcement angle on the big sides, and a dowel/tube between these two Xs in case other suitcases/luggage piled on top of laid flat Brompton (in its cage, in padded panels (scavenged cardboard?) in its Dimpa? cover?
The dismantled cage could then be carried on front rack /in luggage at destination? Has any one made/seen a DIY or even a commercial variant?
NB you are not suffering from deja vu, I just moved this post to its own thread